Probe-able voltage contrast test structures

ABSTRACT

Test structures and method for detecting defects using the same. A probe-able voltage contrast (VC) comb test structure that includes first, second and third probe pads, a comb-like structure including grounded tines, floating tines between the grounded tines, switching devices coupled with an end portion of each floating tine, and connecting the floating tines to the second probe pad, and the third probe pad being a control pad which controls the switching devices. A probe-able VC serpentine test structure that includes first, second, third and fourth probe pads, a comb-like structure including grounded tines, floating tines between the grounded tines and each floating tine connected together between the second and third probe pads, switching devices connected to an end portion of each floating tine and connecting the floating tines to the second and third probe pads, and the fourth probe pad being a control pad which controls the switching devices.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/539,732,filed Aug. 12, 2009, now U.S. Pat. No. 8,350,583, thedisclosure of which is incorporated by reference herein in its entirety.

BACKGROUND

This invention relates to test structures for semiconductor fabrication,and more particularly to probe-able voltage contrast test structures forelectrical testing and voltage contrast inspection, and a method fordetecting defects using the same.

Mask area (space on the reticle) is a precious resource used duringtechnology development and manufacturing of integrated circuits. Masksets may cost 1 million dollars or more. During process development awide range of test structures for characterizing the yield andfunctionality of different circuit components must incorporated on eachmask set. In addition, design IP must also be included to test out thebuilding blocks for ICs that will be manufacturing for sale. Duringmanufacturing, primarily chips that will be sold consume the entire maskarea. Generally there is not enough room on a mask set to accommodateall the test structures and other designs that could provide value Twoclasses of test structures often included on mask sets are probe-abletest structures (e.g., combs and serpentine patterns) which are used totest for shorts and opens using electrical probes, and voltage contrasttest structures which are used in line with a scanning electronmicroscope (SEM). The voltage contrast test structures provide feedbackon defectivity at a level shortly after defect formation. The exactlocation of each defect is also isolated using this technique.Probe-able test structures are important because they enable a verylarge area to be tested quickly. Voltage contrast inspection is timeconsuming and so many wafers go without inspection. A greater number ofwafers can be probed. Also using electrical probes, the exact resistancecan be measured.

Probe-able test structures and voltage contrast test structures aredifferent in structure. Probe-able test structures require large probepads, which are connect to two or more electrical nodes in thestructure. FIGS. 1A and 1B are diagrams illustrating conventionalprobe-able comb and serpentine structures, respectively. As shown inFIG. 1A, a conventional probe-able comb test structure 100 is provided.The conventional probe-able comb test structure 100 includes a pluralityof probe pads 101 and 102 respectively connected to comb-like structures103 and 104. In FIG. 1B, a conventional probe-able serpentine teststructure 110 is provided. The conventional probe-able serpentine teststructure 110, includes a plurality of probe pads 111 and 112 and asingle meandering metal or wire 113 connected therebetween.

On the other hand, a voltage contrast test structure requires smallerelectrical nodes for efficient defect isolation. FIG. 2 illustrates aconventional voltage contrast test structure 200. The conventionalvoltage contrast test structure 200 includes a grounded comb 201including a plurality of grounded tines 202, and a plurality of floatingtines 203 where each floating tine 203 is in between each grounded tine202. These floating tines 203 are independent to allow defect isolation.To test for a short, end portions of the floating tines 203 are scannedin a scan area 204 and if there is a bridge from any one of the floatingtines 203 to any of the grounded tines 202, the respective floating tine203 becomes grounded.

The masking area has a limited amount of space. The probe-able teststructures and the voltage contrast test structure typically areallocated in separate areas since they are designed differently.Therefore, a large amount of space within the masking area is used toaccommodate these test structures.

SUMMARY

Embodiments of the present invention provide probe-able voltage contrastcomb and serpentine test structures which save space within the maskingarea. These test structures may be inspected for defects using voltagecontrast inspection where the exact defect location may be isolatedand/or they may be electrically probed. In addition to the saving space,the data from these techniques can be compared to ensure each techniqueis performing accurately and to more thoroughly characterize thedefectivity.

According to an embodiment of the present invention, a test structurefor detecting defects within integrated circuits is provided. The teststructure includes first, second and third probe pads, the first probepad being connected to ground, a comb-like structure including aplurality of grounded tines connected to the first probe pad, and aplurality of floating tines, each floating tine provided in between thegrounded tines. The test structure further includes a plurality ofswitching devices, each switching device coupled with an end portion ofeach floating tine, and connecting the floating tines to the secondprobe pad, and the third probe pad is a control pad connected to theplurality of switching devices, which controls on and off states of theswitching devices during testing.

According to another embodiment of the present invention, a teststructure for detecting defects within integrated circuits is provided.The test structure includes first, second, third and fourth probe pads,the first probe pad being connected to ground, a comb-like structureincluding a plurality of grounded tines and connected to the first probepad, and a plurality of floating tines, each floating tine provided inbetween the grounded tines and each floating tine connected togetherbetween the second and third probe pads. The test structure furtherincludes a plurality of switching devices, each switching deviceconnected to an end portion of each floating tine and connecting thefloating tines to the second and third probe pads, and the fourth probepad is a control pad connected to the plurality of switching devices,which controls on and off states of the switching devices duringtesting.

According to another embodiment of the present invention, a method ofdetecting shorts using a test structure having first and second probepads and a plurality of grounded tines connected with the first probepad. The method includes pulling a gate of each of the plurality ofswitching devices down via a resistor, to turn off the plurality of theswitching devices, disconnecting the plurality of floating tines fromeach other and the second probe pad. The method further includesscanning the test structure via an electron beam inspection tool todetect floating tines in the plurality of grounded tines, and groundedtines in the plurality of floating tines.

According to yet another embodiment of the present invention, a methodof detecting opens and shorts using a test structure having first,second and third probe pads and a plurality of grounded tines connectedwith the first probe pad is provided. The method includes pulling a gateof each of the plurality of switching devices down via a resistor toturn off the switching devices, isolating the plurality of floatingtines between the second probe pad and the third probe pad. The methodfurther includes scanning the test structure via an electron inspectiontool and detecting opens in the grounded tines and shorts in thefloating tines.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with advantagesand features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIGS. 1A and 1B are diagrams illustrating conventional probe-able comband serpentine test structures, respectively.

FIG. 2 is a diagram illustrating a conventional voltage contrast (VC)comb test structure.

FIG. 3 is a diagram illustrating a probe-able VC comb test structurethat can be implemented within embodiments of the present invention.

FIG. 4 is a diagram illustrating an example of the probe-able VC combtest structure shown in FIG. 3, having a short detected in-line with avoltage contrast inspection that can be implemented within embodimentsof the present invention.

FIG. 5 is a diagram illustrating an example of the probe-able VC combtest structure shown in FIG. 3, during an electrical test operation thatcan be implemented within alternative embodiments of the presentinvention.

FIG. 6 is a diagram illustrating a probe-able VC serpentine teststructure that can be implemented within embodiments of the presentinvention.

FIGS. 7A and 7B are flowcharts respectively illustrating a method fordetecting shorts using the probe-able VC comb test structure shown inFIG. 3 during in-line testing via an inspection scanning electronmicroscope (SEM) and end of line testing via a prober, in accordancewith an embodiment of the present invention.

FIGS. 8A and 8B are flowcharts respectively illustrating a method fordetecting opens and shorts using the VC serpentine test structure shownin FIG. 6 during in-line test via an inspection SEM and end of linetesting via a prober, in accordance with an embodiment of the presentinvention.

The detailed description explains the preferred embodiments of theinvention, together with advantages and features, by way of example withreference to the drawings.

DETAILED DESCRIPTION

Turning now to the drawings in greater detail, it will be seen that inFIG. 3, there is a probe-able VC comb test structure according to anembodiment of the present invention. As shown in FIG. 3, a probe-able VCcomb test structure 300 includes a plurality of probe pads (i.e., firstand second probe pads 305 and 310), wherein the first probe pad 305 isconnected to ground. The test structure 300 further includes a comb-likestructure (i.e., a grounded comb 315) which includes a plurality ofgrounded tines 320. The test structure 300 further includes a pluralityof floating tines 325. Each floating tine 325 is provided in betweeneach grounded tine 320 for detection of shorts (as depicted in FIG. 4,for example).

According to an embodiment of the present invention, the test structure300 further includes a plurality of active switching devices 330corresponding to each of the plurality of floating tines 325. Eachswitching device 330 is coupled with an end portion of each floatingtine 325. The switching devices 330 connect the plurality of floatingtines 325 to the probe pad 310, for selectively connecting the floatingtines 325 together. According to an embodiment of the present invention,the switching devices 330 may each include an n-type field effecttransistor (nFET). That is, each floating tine 325 is connected to arespective switching device 330 which is in turn connected to probe pad310 such that all of the floating tines 325 are connected together andto probe pad 310. According to an embodiment of the present invention,the width of each of the floating tines 325 is substantially similar toone another.

According to an embodiment of the present invention, the probe-ablevoltage contrast test structure 300 further includes a third probe pad(i.e., a control pad 335) coupled to the switching devices 330. Thecontrol pad 335 controls an on/off state of the switching devices 330.The probe-able voltage contrast test structure 300 further includes aresistor 340 wherein the control pad 335 is connected through theresistor 340 to ground. Each gate 332 of the switching devices 330 areconnected to the control pad 335 and in turn connected to ground via theresistor 340. According to the current embodiment, the switching devices330 are forced off by the resistor 340 during voltage contrastinspection because the control pad 335 is connected through the resistor340 to ground. Therefore, each floating tine 325 is isolated. A chargeis induced on the floating tines 325 to detect any shorts. The switchingdevices 330 are used to transform the floating tines 325 (i.e.,electrical nodes) of the VC comb test structure 300 or other teststructure into a single electrical node which is connected to the probepad 310. An example of the detection of a short will now be discussedbelow with reference to FIG. 4.

FIG. 4 is an example of the probe-able VC comb test structure 300 shownin FIG. 3, having a short during voltage contrast inspection. As shownin FIG. 4, during the voltage contrast inspection, the control pad 335controls the switching devices 330 to be in an off state. That is, thegates 332 of the switching devices 330 are pulled to ground through theresistor 340 so that the switching devices 330 are off during voltagecontrast inspection. Therefore, each floating tine 325 is isolated.According to another embodiment of the present invention, to ensure thatthe switching devices 330 stay off during inspection, the gate 332 ofeach switching device 330 may be tied to ground or a virtual groundthrough a resistor.

During the in-line VC inspection, the test structure 300 is scanned witha scanning electron microscope (SEM) and the SEM induces a charge on allthe electrically floating tines 325 while the grounded tines 320 remainin a grounded state. Further, as shown, when a short 350 exists betweena respective floating tine e.g., a floating tine 325 a, for example, anda grounded tine 320 adjacent to the respective tine 325 a, therespective tine 325 a becomes grounded and turns bright. The groundedtines 320 emit more electrons than the floating tines 325 under electronextraction conditions thereby causing them to appear brighter than thefloating tines 325. The other floating tines 325 remain dark.

FIG. 5 is an example of the probe-able VC comb test structure 300 shownin FIG. 3, during an electrical test operation. During probing, theswitching devices 330 are turned on by pulling the control pad 335‘high’ and all of the floating tines 325 which are normally isolated areshorted together and connected to the probe pad 310 whereas the groundedtines 320 are connected to the probe pad 305. According to thisembodiment, probe pad 310 is checking for shorts and when a short 350exists, all of the tines 325 become grounded. During probing, thecontrol pad 335 connected to the gates 332 of all the switching devices330 is biased to a positive voltage greater than a threshold voltage,which may be approximately 0.15 volts (V). The control pad 335 is ontherefore turning the switching devices 330 on during probing. All thenormally floating tines 325 are then shorted into “one large bottomcomb”. The same short defect now causes the entire bottom comb to beshorted to the grounded comb 315 (as shown in FIG. 5, for example).

FIG. 6 is a diagram illustrating a probe-able voltage contrastserpentine test structure 600 which tests for opens which may occur dueto missing conductive material defects. The test structure 600 includesa plurality of probe pads (i.e., first, second and third probe pads)605, 610, and 615. The first probe pad 605 is connected to ground. Thetest structure 600 further includes a grounded comb 617 including aplurality of grounded tines 620 which are connected to the first probepad 605. The test structure 600 further includes a plurality of floatingtines 625. Each floating tine 625 is provided in between each groundedtine 620 and each floating tine 620 may be connected together betweenthe second and third probe pads 610 and 615.

According to an embodiment of the present invention, the test structure600 further includes a plurality of switching devices 630. Eachswitching device 630 is connected to an end portion of each floatingtine 625 and connects the floating tines 625 to the second and thirdprobe pads 610 and 615. According to an embodiment of the presentinvention, some of the switching devices 630 (i.e., switching devices630 a, 630 b, 630 c and 630 d) are connected to one end portion ofrespective floating tines 625 and the remaining switching devices 630(i.e., switching devices 630 e, 630 f and 630 g) are connected toopposite end portions of respective floating tines 625. According to anembodiment of the present invention, the switching devices 630 may benFETs.

The probe-able voltage contrast serpentine test structure 600 furtherincludes a fourth probe pad (i.e., a control pad 635) connected to theplurality of switching devices 630, which controls on and off states ofthe switching devices 630. According to an embodiment of the presentinvention, gates 632 of the switching devices 630 are all connected tothe control pad 635. During voltage contrast inspection, the switchingdevices 630 are in an off state and connected to ground through aresistor 640 connected to the control pad 635, and opens are detected inthe grounded tines 620 of the grounded comb 617. During probing, theswitching devices 630 are switched to an on state by the control pad 635and opens are detected in the floating tines 625 from the probe pad 610to the probe pad 615 by measuring the resistance from the probe pad 610to the probe pad 615.

FIGS. 7A and 7B are flowcharts respectively illustrating a method fordetecting shorts using the test structure shown in FIG. 3 during in-linetesting via an inspection SEM and end of line testing via prober.

In FIG. 7A, in-line testing is performed via an inspection SEM. In thisfigure, the method is performed using a test structure having first andsecond probe pads and a plurality of grounded tines connected with thefirst probe pad. At operation 700, a gate of each of the plurality ofswitching devices is pulled down via a resistor to turn off theplurality of switching devices, disconnecting the plurality of floatingtines from each other and the second probe pad. From operation 700, theprocess moves to operation 705 where the test structure is scanned viaan electron beam inspection tool to detect floating tines in theplurality of grounded tines and grounded tines in the plurality offloating tines.

In FIG. 7B, end of line testing is performed via a prober. According toan embodiment of the present invention, at operation 710, a control padis forced to a positive voltage greater than a threshold voltage to turnon the switching devices during probing, and to connect the isolatedfloating tines together and to the second probe pad. From operation 710,the process moves to operation 715, where a resistance is measuredbetween the first and second probe pads to detect any shorts from thefloating tines to the grounded tines via a prober.

According to an embodiment of the present invention, the thresholdvoltage is approximately 0.15 volts (V).

FIGS. 8B and 8B are flowcharts respectively illustrating a method fordetecting opens and shorts using the test structure shown in FIG. 6during in-line testing via an inspection SEM and end of line testing viaa prober.

In FIG. 8A, in-line testing is performed via an inspection SEM. In thisfigure, the method is performed using a test structure having first,second and third probe pads and a plurality of grounded tines connectedwith the first probe pad. At operation 800, a gate of each of theplurality of switching devices is pulled down via a resistor to turn offthe switching devices, isolating the plurality of floating tines betweenthe second probe pad and the third probe pad. From operation 800, theprocess moves to operation 805 where the test structure is scanned viaan electron beam inspection tool to detect opens in the grounded tinesand shorts in the floating tines.

In FIG. 8B, end of line testing is performed via a prober. According toan embodiment of the present invention, at operation 810, a control padis forced to a positive voltage greater than a threshold voltage to turnon the switching devices during probing, and to connect the floatingtines together and to the second and third probe pads. From operation810, the process moves to operation 815, where a resistance between thesecond and third probe pads is measured to detect opens in the floatingtines and a resistance between the first and second probe pads ismeasured to detect shorts from the floating tines to the grounded tinesvia a prober.

According to an embodiment of the present invention, the same area isused for testing shorts and opens. Thus, the present invention providesthe advantage of saving masking space. Further, the data generated fromboth these techniques may be compared to ensure that each technique isperforming properly.

While the preferred embodiment to the invention has been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

What is claimed is:
 1. A test structure comprising: first, second, thirdand fourth probe pads, the first probe pad being connected to ground; acomb-like structure including a plurality of grounded tines andconnected to the first probe pad; a plurality of floating tines, eachfloating tine provided in between the grounded tines and each floatingtine connected together between the second and third probe pads; aplurality of switching devices, each switching device connected to anend portion of each floating tine and connecting the floating tines tothe second and third probe pads, such that each floating tine isconnected to switching device at both ends thereof, so as to form aserpentine-like structure of serially connected floating tines betweenthe second and third probe pads; and the fourth probe pad being acontrol pad connected to the plurality of switching devices, whichcontrols on and off states of the switching devices during testing. 2.The test structure of claim 1, further comprising a resistor wherein thecontrol pad is connected through the resistor to ground.
 3. The teststructure of claim 2, wherein the switching devices are n-type fieldeffect transistors.
 4. The test structure of claim 3, wherein gates ofthe plurality of switching devices are connected to the control pad. 5.The test structure of claim 4, wherein during voltage contrastinspection, the switching devices are in an off state, and the teststructure tests for opens in the grounded tines.
 6. The test structureof claim 5, wherein during probing, the switching devices are switchedto an on state by the control pad and the test structure tests for opensin the floating tines between the second probe pad and the third probepad.